1. Field of the Invention
This invention relates to the fabrication of devices and, in particular, the fabrication of devices that include a silicon oxide region.
2. Art Background
In the fabrication of devices such as semiconductor devices, a variety of material layers is sequentially formed and processed on a substrate. (For the purpose of this disclosure, the substrate includes a bulk material such as 1) a pyroelectric, e.g., LiNbO.sub.3 or 2) a semiconductor, e.g., silicon, body and, if present, various regions of materials such as dielectric materials, metallic materials, and/or semiconductor materials). Often, one of the material regions utilized in this fabrication procedure includes a silicon oxide, i.e., a material nominally represented by the formula SiO.sub.n where 0&lt;n.ltoreq.2. For example, silicon oxide regions are utilized as passivating layers, as electrical insulation between conducting layers, e.g., metal layers, and as a cap for doped semiconductor layers to limit unacceptable dopant migration during subsequent processing.
A silicon oxide is often deposited on a non-planar substrate surface having a plurality of steps (6 in FIG. 1). It is desirable that the deposited silicon conformally coat this non-planar surface. If a conformal silicon oxide layer is not achieved, an irregular coating, 18, forms over the underlying steps, 12. If deposition is continued, voids, 10, as shown in FIG. 2, are often produced. An irregular coating such as shown in FIG. 1 is, in many situations, unacceptable because a non-planar surface degrades the resolution of subsequent photolithography. Voids such as shown in FIG. 2 are even less desirable because etching and dielectric properties will be nonuniform. In either case, lack of planarity generally produces difficulties in subsequent processing. Therefore, it is very desirable to produce a conformal coating. (Conformance is measured by two ratios, i.e., (1) the ratio between dimension s (FIG. 3) and dimension t and (2) the ratio between dimension b and dimension t. Both ratios should be in the range 0.9 to 1.0 for a layer to be considered conformal.)
Although silicon oxide conformal coatings are very difficult to produce, one process generally denominated, the TEOS (tetraethoxysilane) process, leads to a silicon oxide region having ratios of approximately 0.9 for 1.0 .mu.m-dimension steps. This process involves flowing tetraethoxysilane over a heated deposition substrate and thus causing pyrolysis of the compound with the resulting formation of silicon oxide. However, the decomposition of the tetraethoxysilane must be performed at temperatures of approximately 700 degrees C. or higher to achieve an acceptable silicon oxide deposition rate, i.e., a rate greater than 10 Angstroms/minute. (Phosphorus oxide doping of silicon oxide formed from TEOS allows deposition at temperatures down to 650 degrees C.). However, device structures, e.g., LiNbO.sub.3 /silicon interfaces, doped silicon regions, and aluminum/silicon interfaces, which cannot tolerate nominal exposure to temperatures above 500 degrees C. are common, and this sensitivity severely limits the usefulness of the TEOS process.
Lower temperature processes for depositing silicon oxides are available. Exemplary of such processes is the reaction of silane and oxygen at about 400 degrees C. Although temperatures of 400 degrees C. do not substantially affect aluminum or doped semiconductor materials, the resulting silicon oxide region contains particulate matter and is extremely non-conformal, exhibiting ratios less than 0.5. Not only are the previously discussed difficulties relating to non-conformal surfaces present, but also the particulate matter causes non-uniform etching and electrical properties. Thus, an acceptable low temperature procedure for forming a conformal region of a silicon oxide has not been reported.